Japanese Patent Laid-Open No. 2011-45839 (Patent Literature 1), for example, describes a technique of taking an image of a floc flocculation state in a stock solution and adjusting the addition amount of flocculant and the rotation speed of a flocculant stirrer according to the related art. This technique includes: an imaging unit that takes an image of flocculated flocs, a conversion unit that converts a luminance signal of the image of flocculated floes into an electric signal, binarizes an image from the electric signal, and generates a binary image; a calculation unit that calculates a flocculated floe area displayed on the binary image; a comparison unit that compares the flocculated floc measurement area and a flocculated floc reference area; a control unit that controls the rotation speed of a flocculant supply pump and the flocculant stirrer according to the comparison result; and an output unit that outputs a warning according to the comparison result. The calculation unit calculates multiple flocculated floc areas beforehand with a certain flocculant addition ratio, calculates an average flocculated floc area that is the mean value of the multiple flocculated floc areas, and calculates the flocculated floc reference area based on the average flocculated floc area.
According to the flocculated floc area of flocculated flocs delivered from a flocculating-mixing tank, this technique controls the addition ratio of the flocculant in sludge to be supplied to the flocculating-mixing tank and controls the rotation speed of the flocculant stirrer that slowly stirs the flocculant.
Japanese Patent Laid-Open No, 2010-247151 (Patent Literature 2) describes a technique of automatically identifying a sludge flocculation state. This technique includes the steps of: charging a predetermined amount of flocculant into sludge; taking an image of the sludge flocculation state; compressing digital image data on the image of the sludge flocculation state; and comparing the amount of the compressed digital image data with a predetermined threshold value. In the step of comparison, when the amount of the compressed digital image data is smaller than the predetermined threshold value, the amount of charged flocculant is increased or reduced.